Telephone apparatus connected to a telephone line must, in general, present certain d.c. characteristics and a defined a.c. impedance in the signal band to the telephone line. The d.c. characteristics and the a.c. impedance specifications differ between countries.
The apparatus must also include a device for reducing the level, in its receive path, of a signal it transmits. This device is inevitably sensitive to the impedance presented to the apparatus by the telephone line. Ideally, the apparatus would be designed to provide the best rejection of its own transmitted signals for the impedance presented by the telephone line in use, which is termed the best hybrid balance. This has been approximated in previous circuits, for example the Motorola TCA3386 and TCA3388 circuits, by using a weighted sum of two impedance networks to define a hybrid balance impedance. This approach ha.sub.s the disadvantages that the gain of the receive path becomes frequency dependent, and that extra impedance networks are necessary beyond that required to define the impedance presented to the telephone line. These disadvantages become significant in programmable equipment designed for portable use in a number of countries because different networks will be required for each country.